High impulse solid propellant composition



United States Patent Ofifice 3,053,708 Patented Sept. 11, 1962 3,053,708HIGH IMPULSE SULlD PROPELLANT COMPOSHTHON Robert L. Hall and Otho l).Ratliff, McGregor, Tex., as-

signors to Phillips Petroleum Company, a corporation of Delaware NoDrawing. Filed Feb. 12, 1959, Ser. No. 792,916

7 Claims. (Cl. 149-19) This invention relates to high impulse solidpropellant j compositions containing elemental silicon. In one aspect Ithe invention relates to a solid propellant containing elemental siliconas a high impulse ingredient and also containing elemental aluminum asan ignition aid for the silicon.

High energy fuel components such as aluminum, magnesium, boron, andsilicon have been proposed for use as ingredients of solid propellantcompositions to improve the specific impulse of such propellantcompositions. These high energy fuels can be combined with solidpropellant compositions wherein a solid oxidizer, such as ammoniumnitrate or ammonium perchlorate, is combined with a binder materialwhich also acts as the fuel component for the propellant composition.

Recently it has been discovered that superior solid propellantcompositions are obtained comprising a solid, in organic, oxidizingsalt, such as ammonium nitrate or ammonium perchlorate, and a rubberybinder material, such as natural rubber or a copolymer of butadiene, anda vinyl pyridine or other substituted heterocyclic nitrogen basecompound which, after incorporation, is cured by a quaternizationreaction for a vulcanization reaction. Solid propellant compositions ofthis nature and a process for their production are disclosed and claimedin copending application Serial No. 284,447, filed April 25, 1952, by W.B. Reynolds and I. E. Pritchard, now US. Patent No. 3,003,861, issuedOctober 10, 1961.

Elemental silicon is considered a superior fuel compoent for a solidpropellant charge because of the great quantity of heat released uponthe formation of silicon dioxide; however, attempts to utilize finelydivided elemental silicon as a high-energy fuel in solid propellantcharges has not been successful and it has been determined that thesilicon is usually unchanged after combustion of the propellant charge.Therefore, the silicon provides no thrust to the reaction and thespecific impulse of the propellant composition is not improved.

It is an object of this invention to provide a solid propellantcomposition wherein finely divided elemental silicon is utilized toincrease the specific impulse properties of the propellant composition.It is also an object of this invention to provide a solid propellantcomposition containing finely divided elemental silicon wherein finelydivided aluminum is utilized as an ignition aid for the silicon. It is afurther object of this invention to provide a method for the ignition offinely divided silicon in a solid propellant composition. Other objectsand advantages of the invention will be apparent to one skilled in theart upon study of the present disclosure.

We have discovered that the high impulse characteristics of elementalsilicon can be imparted to a solid propellant composition containingsilicon by the addition thereto of a sufiicient amount of aluminum toinsure the ignition of the silicon. Solid propellant compositions,comprising ammonium nitrate as the oxidizer and a rubbery copolymer asthe binder materials, have been prepared containing powdered aluminumand the aluminum has ignited with the conventional igniters used forsuch propellant compositions. We have discovered that in suchformulations wherein finely divided or powdered silicon is employed, thesilicon can be ignited by incorporating in the composition powderedaluminum in an amount of about 1 up to about 60 weight percent ofaluminum based on the total of aluminum and silicon. Rapid ignition ofthe silicon is also facilitated if the igniter employed contains in theignition charge a substantial amount of powdered aluminum so that thetemperature of the burning gases is quickly raised to above the ignitiontemperature of silicon. When from about 1 to about 4 or 5 weight percentof aluminum is used, with respect to the total of silicon and aluminum,the maximum temperature in the combustion chamber is not attained forseveral seconds, for example, 2 to 3 seconds, Whereas if from about 6 toabout 60 weight percent of aluminum is used, with respect to the total.of aluminum and silicon, then the temperature rise in the combustionchamber is quite rapid and maximum temperature is achieved almostinstantly.

Ammonium nitrate is the preferred oxidizer for the reason that ammoniumnitrate contributes more oxygen per unit weight than do other solidoxidizing agents such as ammonium perchlorate. The amount of oxidizerused is that required to convert the aluminum and the silicon to theoxide; the carbon of the binder to carbon monoxide; and the hydrocarbonof the binder to hydrogen and carbon monoxide. While any of the rubberycopolymer binder compositions of the propellant art can be used as thebinder and a portion of the fuel of the propellant composition of ourinvention, the preferred compositions for the binder component, becauseof superior physical properties and impulse characteristics, aredisclosed in copending application Serial No. 730,755, filed April 21,1958, by F. R. Gessner, Jr.

The following tabulation discloses a typical formulation of a propellantcomposition containing silicon and aluminum.

Table I Ingredient: Weight percent Butadiene methylvinylpyridinecopolymer 3 00 6.67 Plasticizer 0 50- 2.00 Silicon and aluminum combined15.00-32.00 Carbon black 0.50- 1.33 Ammonium nitrate 60.00-81.00

The various ingredients making up the total composition are thoroughlyincorporated by mixing all of the ingredients so that a composition isobtained wherein the binder forms the continuous phase. The silicon andaluminum are utilized as finely divided or powdered materials and can beadded separately or mixed together before being added to thecomposition.

The composition is then formed into the desired shape by a molding orextrusion operation and the propellant composition is cured, forexample, for 24 hours at F.

The silicon and the aluminum can be reduced to finely divided form byany of the known methods of pulverization. The aluminum is resistant topulverization by ordinary methods since it is a soft, tough metalwhereas silicon, being extremely brittle, is amenable to ordinarycrushing operations and is readily reduced to a finely divided statesuch as a powder. reducing the aluminum to powdered form is to preparean alloy of aluminum and silicon in the desired ratio in which case anextremely brittle alloy results which is readily reduced to finelydivided form by ordinary crushing methods. disintegrate to powdered formupon cooling due to stresses set up therein and due to the extremelybrittle nature of the alloy.

The rubbery binders of the solid rocket propellant compositions includenatural rubber and synthetic rubbers such as butadiene-styrene;polybutadiene; silicon rubber,

polysulfide rubber; copolymers of conjugated dienes with A preferredmethod 'for In some cases these alloys spontaneously 3 polymerizableheterocyclic nitrogen bases of the pyridine and quinoline series; andthe like.

The conjugated dienes which can be employed are those containing from 4to 6 carbon atoms per molecule and include 1,3-butadiene, isoprene,2-methyl-l,3-butadiene, and the like. Various alkoxy, such as methoxyand ethoxy and cyano derivatives of these conjugated dienes, are alsoapplicable. Thus, other dienes, such as phenyl butadieue,2,3-dimethyl-1,3-hexadiene, Z-methoxy- B-ethyIbutadiene,2-ethoxy-3-ethyl-l,3-hexadiene, 2-cyano- 1,3-butadiene, are alsoapplicable in the preparation of the polymeric binders of thisinvention.

Instead of using a single conjugated diene, a mixture of conjugateddienes can be employed. Thus, a mixture of 1,3-butadiene and isoprenecan be employed as the conjugated diene portion of the monomer system.

The polymerizable heterocyclic nitrogen bases which are applicable forthe production of the polymeric materials are those of the pyridine,quinoline, and isoquinoline series which are copolymerizable with aconjugated diene and contain one, and only one substituent wherein R iseither hydrogen or a methyl group. That is, the substituent is either avinyl or an alpha-methylvinyl(isopropenyl) group. Of these, thecompounds of the pyridine series are of the greatest interestcommercially at present. Various substituted derivatives are alsoapplicable but the total number of carbon atoms in the groups attachedto the carbon atoms of the heterocyclic nucleus should not be greaterthan because the polymerization rate decreases somewhat with increasingsize of the alkyl group.

Solid inorganic oxidizing salts which are applicable in the solid rocketfuel compositions of this invention include ammonium, alkali metal, andalkaline earth metal salts of nitric, perchloric, and chloric acids, andmixtures thereof. Ammonium nitrate and ammonium perchlorate are thepreferred oxidants for use in the solid rocket fuels of this inventionwith ammonium nitrate being especially preferred because of the greateramount of oxygen contributed per unit weight by the ammonium nitrate.Other specific oxidant which can be used include sodium nitrate,potassium perchlorate, lithium chlorate, calcium nitrate, bariumperchlorate, and strontium chlorate. Mixtures of oxidants are alsoapplicable. In the preparation of the solid rocket fuel compositions,the oxidants are powdered to sizes preferably 10 to 300 microns averageparticle size. The amount of solid oxidant employed is usually a majoramount of the total composition and is generally in the range between 50and 90 percent by weight of the total mixture of oxidant and binder. Ifdesired, however, less than 50 percent by weight of the oxidant can beused.

Combustion rate catalysts can be used if desired and those applicable inthe propellant composition of the invention include ammonium dichromate,metal ferrocyanides and metal ferricyanides. The complex metal cyanidesare preferred. Ferric ferrocyanides, such as Prussian, Berlin, Hamburg,Chinese, Paris, and milori blue, soluble ferric ferrocyanide, such assoluble Berlin or Prussian blue which contains potassium ferricferrocyanide, and ferric ferrocyanide which has been treated withammonia, are among the materials which can be used. Ferrousfern'cyanide, Turnbulls blue, is also applicable. A particularlyeffective burning rate catalyst is milori blue which is pigment similarto Prussian blue, and is prepared by the oxidation of a paste ofpotassium ferrocyanide and ferrous sulfate. Other metal compounds suchas nickel and copper ferrocyanides can also be employed. The amount ofcombustion or burning rate catalyst will usually be about 0.25 to about12 parts by weight per 100 4 parts of oxidant and binder. The catalystcan be omitted entirely if desired.

Reinforcing agents include carbon black, wood flour, lignin, and variousreinforcing resins such as styrene-divinylbenzene, methyl andacrylate-divinylbenzene, acrylic acid-styrene-divinylbenzene, methylacrylate-acrylic aciddivinylbenzene, and phenyl-formaldehyde resins. Thereinforcing agent is usually used in an, amount in the range of about 10to about 50 parts by weight per parts by weight of rubbery material. Thereinforcing agent can be omitted if desired.

In general, any rubber plasticizer can be employed in these bindercompositions. Material such as Pentaryl A (amyl biphenyl), Paraflux(saturated polymerized hydrocarbon), Circosol 2XH (petroleum hydrocarbonsoftener having a specific gravity of 0.94 and a Saybolt Universalviscosity at 100 F. of about 2000 seconds),di-(butoxyethoxyethyl)formal, and dioctyl phthalate are suitableplasticizers. Liquid polybutadiene and aromatic hydrocarbon oilsresulting from the distillation of petroleum fractions are preferredplasticizers because they are particularly effective in rendering thecomponents of the composition manageable and are entirely consumed asfuel. An aromatic residual oil having an API gravity at 60 F. of about10 to about 13.5 has been found particularly effective. The amount ofplasticizer used will be only that required to render the rubberymaterial manageable during incorporation of the oxidizer and extrusionof the product, ordinarily, about 0.5 to about 10 weight percent of thetotal propellant composition.

The various ingredients of the rocket fuel composition can be mixed on aroll mill or in an internal mixer such as a Banbury, or a Baker-Perkinsdispersion blade mixer can be employed. The binder forms the continuousphase in the finished propellant composition with the oxidant as thediscontinuous phase. The propellant is usually cured as a temperature ofabout 70 to about 250 F. for about 6 to about 72 hours.

The following specific embodiments of the invention are exemplary andare not to be construed as limiting the invention but are presented tofacilitate an understanding of the invention.

EXAMPLE I The following tabulation discloses one preferred formulationof a propellant composition containing silicon and aluminum withammonium nitrate as the oxidizer.

Table II SILICON-CONTAINING SOLID PROPELLANT Ingredient: Weight percentButadiene-methylvinylpyridine copolymer 5.50 Furnace carbon black 1.00Liquid polybutadiene 2.00 Magnesium oxide 0.25 Silicon 25.00 Aluminum2.25 Ammonium nitrate 64.00

The copolymer, liquid butadiene, carbon black, magnesium oxide areblended together and the silicon and aluminum in the form of a powderedalloy are then added to the blend. The silicon and aluminum, in theproportion indicated in the table, can be alloyed and the resultingalloy crushed to powder form having an average particle size of about 5microns. The ammonium nitrate is then incorporated into the above blendof materials to form the finished composition which is extruded into theproper grain forms and is cured for 24 hours at F. The specific impulseof the above solid propellant is about 260 to about 270 seconds.

EXAMPLE II The following tabulation discloses another formulation for apropellant composition.

Table III SILICON-CONTAINING SOLID PROPELLANT Ingredient: Weight percentButadiene-methylvinylpyridine copolymer 3.00 Furnace carbon black 0.65

Aromatic residual oil 0.30 Magnesium oxide 0.05 Aluminum 5.00 Silicon21.00 Ammonium nitrate 70.00

The composition is prepared as hereinbefore disclosed. The specificimpulse is about 260 to about 270 seconds. EXAMPLE III Anotherpropellant composition is shown in the following tabulation.

Table IV SILICON-CONTAINING SOLID PROPELLANT Ingredient: Weight percentButadiene-vinylpyridine copolymer 6.00 Furnace carbon black 1.35Polybutadiene 1.90 Magnesium oxide 0.25 Aluminum 15.00 Silicon 11.00Ammonium nitrate 64.00

The specific impulse of this composition will be lower than those ofExamples I and II and will be about 260 seconds.

EXAMPLE IV Still another propellant composition is shown in thefollowing Table V.

Table V SILICON-CONTAINING SOLID PROPELLANT The specific impulse of thiscomposition will be about 260 to 270 seconds.

The combustion temperatures of the above propellants are sufficientlyhigh to prevent plating of solid alumina or silica on the exhaust nozzlesurfaces.

The solid propellant compositions of this invention are simple incomposition, readily adaptable to present methods of compounding solidpropellants, and are characterized by exceptionally high values ofspecific impulse.

Reasonable variations and modifications are possible within the scope ofthe present disclosure without departing from the spirit and scope ofthe invention.

That which is claimed is:

1. A solid propellant composition comprising about 60 to about 81 weightpercent of a solid inorganic oxidizing salt; about 4 to about weightpercent of a rubber binder material; and about to about 32 weightpercent of a mixture of finely divided silicon and finely dividedaluminum wherein the aluminum is present in an amount sufiicient toraise the temperature of the propellant combustion gases to above theignition temperature of the silicon and is in the range of about 1 up toabout 60 weight percent of the total of aluminum and silicon.

2. A solid propellant composition wherein the hereinafter named silicon,aluminum and binder material constitute from 19 to 40 weight percent ofthe total composition, comprising about 60 to about 81 weight percentammonium nitrate; about 15 to about 32 weight percent of silicon andaluminum combined wherein the aluminum is present in an amountsufiicient to raise the temperature of the propellant combustion gasesto above the ignition temperature of the silicon and is from about 1 toabout 60 weight percent of the total of silicon and aluminum; and

about 4 to about 10 weight percent of a binder comprising a copolymer ofa conjugated diene having from 4 to 6 5 carbon atoms per molecule and atleast one substituted heterocyclic nitrogen base selected from the groupconsisting of pyridine, quinoline, an alkyl substituted pyridine and analkyl substituted quinoline, wherein the total number of carbon atoms inthe nuclear alkyl substituents is not more than 15.

3. A propellant composition according to claim 2 wherein the bindercomprises about 4 weight percent; the combined silicon and aluminumcomprise about 26 Weight percent; and the ammonium nitrate comprisesabout 70 Weight percent of the propellant.

4. A propellant according to claim 2 wherein the binder comprises about9.5 weight percent; the combined aluminum and silicon comprise about26.5 weight percent and the ammonium nitrate comprises about 64 weightpercent of the total propellant.

5. A propellant composition according to claim 2 wherein the bindercomprises about 6 weight percent; the combined aluminum and siliconcomprise about 27.5 weight percent; and the ammonium nitrate comprisesabout 66.5 weight percent of the total propellant.

6. A method for producing a solid rocket propellant composition whichcomprises: intimately admixing about 15 to about 32 parts by weight of amixture of finely divided silicon and finely divided aluminum whereinthe aluminum is about 1 to 60 weight percent of the mixture of siliconand aluminum with about 3 to about 6.67 parts by weight of a rubberybinder comprising a copolymer of a conjugated diene and a polymerizableheterocyclic nitrogen base; incorporating about 60 to about 81 parts byweight of finely divided ammonium nitrate into the resulting mixture;forming the resulting composition into propellant grains; and curing theresulting composition to produce a propellant composition so that thepropellant composition contains a sufficient amount of aluminum to raisethe temperature of the combustion gases to above the ignitiontemperature of the silicon.

7. The method of igniting finely divided silicon contained in a solidpropellant charge comprising a major proportion of a solid inorganicoxidizing salt and a minor proportion of a rubber binder containingfinely divided silicon which method comprises incorporating finelydivided aluminum in the binder in an amount suflicient to raise thetemperature of the propellant combustion gases to above the ignitiontemperature of the silicon and in the range of 1 to 60 weight percent ofthe total of aluminum and silicon.

References Cited in the file of this patent UNITED STATES PATENTS1,054,777 Imperiali Mar. 4, 1913 1,506,322 ONeill Aug. 26, 19242,410,801 Audrieth Nov. 12, 1946 2,416,639 Pearsall Feb. 25, 19472,775,514 Wainer Dec. 25, 1956 2,857,258 Thomas Oct. 21, 1958 2,877,504Fox Mar. 17, 1959 2,926,613 Fox Mar. 1, 1960 FOREIGN PATENTS 742,283Great Britain Dec. 21, 1955 OTHER REFERENCES Zaehringer: ModernPlastics, vol. 34, October 1956, pp. 148-51.

Leonard: Journal American Rocket Society, No. 72, December 1947, pp.14-15.

Chem. and Eng. News, Oct. 7, 1957, pp. 62-3.

1. A SOLID PROPELLANT COMPOSITION COMPRISING ABOUT 60 TO ABOUT 81 WEIGHTPERCENT OF A SOLID INORGANIC OXIDIZING SALT; ABOUT 4 TO 10 WEIGHTPERCENT OF A RUBBER BINDER MATERIAL; AND ABOUT 15 TO 32 WEIGHT PERCENTOF A MIXTURE OF FINELY DIVIDED SILICON AND FINELY DIVIDED ALUMINUMWHEREIN THE ALUMINUM IS PRESENT IN AN AMOUNT SUFFICIENT TO RAISE THETEMPERATURE OF THE PROPELLANT COMBUSTION GASES TO ABOVE THE IGNITION THETEMPERATURE OF THE SILICON AND IS IN THE RANGE OF ABOUT 1 UP TO ABOUT 60WEIGHT PERCENT OF THE TOTAL OF ALUMINUM AND SILICON.